Amino-functional ZIF-8 nanocrystals by microemulsion based mixed linker strategy and the enhanced CO2/N2 separation

High permeability and selectivity are desirable for mixed matrix membranes (MMMs) based CO2 separation. Herein, a microemulsion based mixed linker strategy is proposed to introduce amino groups during the growth of ZIF-8. Compared to post-synthetic modification, this method presents a less reduction in S-BET of ZIF-8, which is beneficial for CO2 adsorption. By controlling the ratio of 2-methylimidazole to 2-aminobenzimidazole, NH2-ZIF-8 with various morphologies and sizes were synthesized and serve as the filler to fabricate the MMMs for CO2 capture. From the scanning electron microscopy (SEM), no obvious agglomerations or defects are observed in ZIF-8/Pebax MMMs, due to the amino groups in ZIF-8 can interact with the amide in Pebax chain, which can improve compatibility between ZIF-8 and Pebax. With the amino groups, NH2-ZIF-8 shows higher affinity with CO2 than that of non-amine-functionalized ZIF-8, leading to an enhancement in the separation performances. When the MMM doped with 6 wt% of NH2-ZIF-8(10), the CO2 permeability reaches up to 163.8 Barrer, which increases by around 53% and 107.6% in contrast with the ZIF-8/Pebax MMM and pure Pebax membrane. Besides, the CO2/N-2 selectivity is enhanced by around 10% and 27%, respectively. These results indicate that amine-functional modification by mixed ligands is a useful way to construct MOFs-based MMMs with high performance for CO2/N-2 separation.